Abstract
Research indicates that neurosteroids are locally synthesized in the central nervous system and play an important modulatory role in nociception. While the neurosteroidogenic enzyme, cytochrome P450 side-chain cleavage enzyme (P450scc), is the initiating enzyme of steroidogenesis, P450scc has not been examined under the pathophysiological conditions associated with peripheral neuropathy. Thus, we investigated whether chronic constriction injury (CCI) of the sciatic nerve increases the expression of P450scc in the spinal cord and whether this increase modulates serine racemase (Srr) expression and D-serine production contributing to the development of neuropathic pain. CCI increased the immunoreactivity of P450scc in astrocytes of the ipsilateral lumbar spinal cord dorsal horn. Intrathecal administration of the P450scc inhibitor, aminoglutethimide, during the induction phase of neuropathic pain (days 0 to 3 post-surgery) significantly suppressed the CCI-induced development of mechanical allodynia and thermal hyperalgesia, the increased expression of astrocyte Srr in both the total and cytosol levels, and the increases in D-serine immunoreactivity at day 3 post-surgery. By contrast, intrathecal administration of aminoglutethimide during the maintenance phase of pain (days 14 to 17 post-surgery) had no effect on the developed neuropathic pain nor the expression of spinal Srr and D-serine immunoreactivity at day 17 post-surgery. Intrathecal administration of exogenous D-serine during the induction phase of neuropathic pain (days 0 to 3 post-surgery) restored the development of mechanical allodynia, but not the thermal hyperalgesia, that were suppressed by aminoglutethimide administration. Collectively, these results demonstrate that spinal P450scc increases the expression of astrocyte Srr and D-serine production, ultimately contributing to the development of mechanical allodynia induced by peripheral nerve injury.
Highlights
Evidence to date shows that activation of spinal N-methyl-Daspartate (NMDA) receptors plays a critical role in the changes in synaptic excitability and the development of neuropathic pain (Woolf and Thompson, 1991; Mao et al, 1992)
We investigated whether: (1) sciatic nerve injury increases the immunoreactivity of P450 side-chain cleavage enzyme (P450scc) in the spinal cord; (2) i.t. administration of the P450scc inhibitor, aminoglutethimide (AMG), suppresses the constriction injury (CCI)-induced mechanical allodynia and thermal hyperalgesia in a mouse model of neuropathic pain; (3) i.t. administration of AMG inhibits the CCI-induced increased expression and/or activation of serine racemase (Srr) and D-serine production in the spinal cord; and (4) exogenous D-serine restores the CCI-induced development of the neuropathic pain that was suppressed by the inhibition of P450scc
To verify whether P450scc activation in the spinal cord is involved in the CCI-induced neuropathic pain, we intrathecally injected the P450scc inhibitor, AMG, during the induction phase and maintenance phase of neuropathic pain
Summary
Evidence to date shows that activation of spinal N-methyl-Daspartate (NMDA) receptors plays a critical role in the changes in synaptic excitability and the development of neuropathic pain (Woolf and Thompson, 1991; Mao et al, 1992). Several reports suggest that peripheral nerve injury increases the expression of Srr in astrocytes and concomitant D-serine production, which play important roles in the functional potentiation of NMDA receptors and the development of peripheral neuropathic pain (Lefèvre et al, 2015; Moon et al, 2015; Choi et al, 2017). Since D-serine can convey nociceptive signaling from glial cells to neurons and change neuronal excitability via activation of NMDA receptors, it is important to investigate the regulatory mechanisms underlying the nerve injury–induced increase in the expression and/or activation of astrocyte Srr and accompanying D-serine production
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